The present invention relates to a Josephson-junction logic circuit and more specifically to a Josephson-junction logic circuit based on SQUID (superconductive quantum interference devices) having nonlinear threshold characteristics.
Conventional Josephson logic gates consisting of multi-junction superconducting quantum interference devices can be divided into two types, i.e., a type with a magnetically coupled gate having magnetically coupled control lines as disclosed in U.S. Pat. No. 3,978,351 and a type with a current-injection gate in which the input current is allowed to flow directly into a loop of the quantum interference device as disclosed in U.S. Pat. No. 4,117,354. In the magnetically coupled type gate, an input current is fed to the magnetically coupled control lines to change the magnetic field coupled with the loop of the SQUID, whereby the critical current of the gate is changed, and the superconductive state is switched to the voltage state. In the magnetically coupled gate, however, each of the input currents that is fed to the magnetically-coupled control lines independently change the critical current. Namely, input sensitivity is low and the operation margin is small. The current injection-type gate, in which the input current is allowed to flow directly into the loop of the gate, utilizes such a phenomenon that the superconductive state is switched to the voltage state when the input current exceeds a threshold value. The current-injection type gate has nonlinear threshold characteristics, and its boundary between the superconductive state and the voltage state changes more rapidly than that of the magnetically coupled gate, making it possible to improve the operation margin. This type of gate, however, has a limitation in the number of inputs. The number of inputs is two in the case of a two-junction interference device of a current-injection type having a large operation margin.
On the other hand, both types of the conventional Josephson-junction logic gates are effective for obtaining a logic sum (OR) or logic product (AND), respectively. However, in order to obtain a functional gate such as an exclusive NOR gate (ENOR gate) by utilizing conventional Josephson-junction logic gates, several Josephson-junction-logic gates are needed.
Further, conventional Josephson logic gates, such as AND gates and OR gates, have different constructions. That is, the OR gate consists of a magnetically coupled gate, and the AND gate having a large operation margin consists of a current-injection type gate. In the Josephson logic circuits which are constructed by using a number of various logic gates, the logic gates must be arrayed in a different manner for each of the circuits. In manufacturing the Josephson logic circuits, therefore, photomasks for patterning electrodes and insulation layers must be prepared in all the manufacturing steps for each of the Josephson logic circuits that are to be produced, thereby making it difficult to correct or modify the Josephson logic circuits.